This invention in general relates to a method for increasing the bulk density of moist coking coals to an optimum level and for controlling and maintaining the bulk density of the moist coals at such optimum levels. The method is particularly adapted to coals which contain between about 6 and 13 weight percent moisture.
The vast reserves of quality metallurgical grade coking coals are being rapidly depleted. Efforts to slow down the depletion of these coals have included upgrading the method of washing and preparing the coals so that the fine raw coal produced from continuous mining can be beneficiated to improve the coal chemistry by the removal of increasing amounts of sulfur and ash; and to recover more of the fine coal particles which in the past were normally wasted to refuse piles. Upgrading of these old processes included adding new steps to improve the beneficiation of the coals. As a result, the moisture content of the coals has increased from an average moisture of about 4 and 5 weight percent to between about 7 to 10 weight percent or more over the past twenty years. Generally, the moisture increase is found in the finer coal sizes shipped from the coal preparation plants.
The beneficiated coal is coked in coke ovens designed to operate at a maximum wall pressure of 13.8 K Pa. The coke produced must not be friable and have sufficient strength to resist degradation during transport and during charging and to support the burden in the blast furnace. To maximize production in the coke ovens with the most efficient use of the energy supplied to the coke ovens and to maintain minimum strain on the walls of the oven, it is essential that the bulk density of the coal be measured and controlled.
All as-received coal contains some moisture for example as little as 2% and as much as 13% or higher. For this reason, it is the usual practice to measure the bulk density of coal as its wet bulk density. The wet bulk density of coal is determined by a standard ASTM D291-60 test procedure or modifications thereof. In the test, a quantity of coal is allowed to fall into a container of known volume. The container filled with coal is weighed. The bulk density is given in weight per unit volume, i.e. pounds per cubic foot or kilograms per cubic meter. The dry bulk density is not determined directly. It is the practice to calculate the dry bulk density of coal from the known wet bulk density and the amount of moisture in the coal.
Both the measured wet bulk density and the calculated dry bulk density of coking coals are of practical importance in the operation of coke ovens. On the one hand, the measured wet bulk density is important in the control of oven filling since for the most part, ovens are filled by gravity feed from hoppers of known volume or by volumetric devices such as screw feeders. In this case, the wet bulk density is needed to control the filling of the oven itself as it is well known that oven underfilling can result in production losses and excessive roof carbon formation. Overfilling of the oven can result in fires, excessive emissions during charging and the production of improperly carbonized coke. On the other hand, the calculated dry bulk density and analytically determined moisture are important in the control of the energy supply to the coke ovens and the production of the coke itself. This relationship between wet and dry bulk density (i.e. the dry bulk density is the weight of dry coal per cubic foot of moist coal) is well known in the art, and should be considered throughout the remainder of this document.
Generally, the bulk density of dry coal is between 53 and 60 pounds per cubic foot (849 and 961 Kg per cubic meter) dependent upon the degree of pulverization and type of coal.
The dry bulk density of the coal decreases as the moisture increases and may be reduced to as low as 38 to 41 pounds per cubic foot (609 to 656 kg per cubic meter, respectively) when the moisture content reaches about 8 weight percent. However, in the range of 8 to 13 weight percent moisture, the dry bulk density of the coals increases slightly to about 42 to 43 pounds per cubic foot (673 to 689 kg per cubic meter). While the dry bulk density does increase with increased moisture content, such additional moisture affects the coking process and additional heat is required to vaporize the moisture. Coke production decreases. Additionally, the coke produced from such moist coal may be weak.
Attempts to offset the deleterious effect of small amounts of moisture on the bulk density of coals have centered on the application of materials which negate the adherence of the water to the coals. It is a known practice to apply small amounts of oil, for example #2 fuel oil, to the surfaces of the coals to increase the bulk density of the coals. Oils so applied do increase the bulk density of coals containing small amounts of moisture, for example between about 2 to 6 weight percent of water. At these moisture levels, the use of fuel oil results in displacement of free water from the surfaces of the coals and thus increases the wet bulk density of the coals. However at relatively high moisture contents, for example about 6 to 8 weight percent and higher, the effect of oil on the bulk density of coals decreases, i.e. large quantities of oil are required for minimal increase in wet bulk density. At moisture contents higher than 8 weight percent, the addition of oil may actually result in a decrease in the wet bulk density of the coals. Then, too, at moisture contents of 2 to 6 percent, the use of the fuel oil does significantly increase the wet bulk density of coal mixes, however at a constant fuel oil treatment rate, small changes in the moisture content can cause wide fluctuations in the wet bulk density of the coals which fluctuations are quite sharp and can result in wet bulk density control problems. Such problems are serious because as noted above, coke ovens are designed to operate at a maximum wall pressure of about 2 pounds per square inch (13.8 K Pa) and coals having very high bulk density may expand excessively during coking resulting in damage to the refractory walls of the oven. The recent rapid rise in the cost of oil products has made the use of oils at high application rates unattractive from a commercial viewpoint.
It has been known to use a wetting agent, for example succinates, or to use combinations of water and a wetting agent, per se, or oil and a wetting agent, as described in U.S. Pat. No. 2,378,420 issued June 19, 1945 to F. A. Lohr et al entitled "Regulating the Bulk Density of Coke Oven Charges." Lohr et al teach that moist coals, i.e. coals containing more than 1 weight percent moisture, can be coated with minute quantities of an oil to increase the wet bulk density of the coals. On the other hand, if water is applied to such coals their wet bulk density decreases as the percentage of moisture increases to between 6 and 8 weight percent. It is therefore possible to adjust the wet bulk density of coals by additions of both oil and water.
Lohr, et al also teach that the wet bulk density of coals can be adjusted by spraying the surfaces of the coals with a free flowing liquid containing a wetting agent, for example sulfonated bi-carboxylic acid (dioctyl sodium sulfosuccinate).
As shown by data and examples in the specification, Lohr et al treat coals containing between 0.9 and 4.5 weight percent moisture. There is no teaching in Lohr, et al that the wet bulk density of coals or blends of coals containing from about 6 weight percent to about 13 weight percent moisture can be increased to an optimum level and that such bulk densities can be controlled and maintained by spraying a suitable non-ionic agent on the surfaces of wet coals.
There is, therefore, a need for a simple, efficient, economical method for increasing the wet bulk density of coking coals which have at least about 6 and as much as about 13 weight percent moisture. The method must be commercially attractive and must increase the wet bulk density of such moist coals to an optimum level. Also, the method must be able to control the wet bulk density of the coals at the optimum level to thereby maintain the productivity of the coke ovens and at the same time have no adverse effects on the properties of the coke or the coking process. In addition, there is a need for a simple method of controlling the wet bulk density of coals wherein the wet bulk density variation is small over a wide range of moisture.
It is the primary object of this invention to provide a method for increasing the wet bulk density of moist coals containing at least about 6 weight percent moisture to an optimum level of between about 47.5 and 49.5 pounds per cubic foot (761 and 793 kg per cubic meter) and maintaining the wet bulk density of such coals at the optimum level, wherein the moist coals are sprayed with an amount of a surfactant equal to about 0.5 to 2.5 pounds (0.227 kg to 1.135 kg) of surfactant per ton (907.2 kg) of coal, the surfactant applied in the form of a 5 to 20 weight percent aqueous dispersion of surfactant and being characterized by the general structural formula, ##STR2## where
R is an alkyl group containing between about 8 and 12 carbon atoms, and
n is an integer between 2 and 30,
an HLB number within the range of between about 4 and 18.
It is an object of this invention to provide a method for increasing, controlling and maintaining the wet bulk density of coals at an optimum level, which method will be commercially attractive.
It is another object of this invention to provide a method for increasing the wet bulk density of wet coals containing between about 6 to 13 weight percent moisture to an optimum level and maintaining the bulk density at such optimum level whereby the productivity of the coke ovens is maximized without deleteriously affecting the properties of the coke or increasing the pressure applied to the walls of the coke ovens above present design limits.
It is still another object of this invention to provide a method for increasing, controlling and maintaining the wet bulk density of coking coals containing more than about 6 weight percent moisture at an optimum level wherein a quantity of a non-ionic oil-soluble or water-dispersible surfactant is sprayed on the surface of the coals, which surfactant will not adversely affect the coking of the coals or the properties of the coke.
It is still another object of this invention to provide a method for energy savings during coking, since the increase in wet bulk density due to the use of the surfactant will increase the amount of coal charged to the ovens but the amount of fuel required to coke this additional quantity of coal will be minimal.
It is still another object of this invention to provide a method for increasing, controlling and maintaining the wet bulk density of coking coals having moisture contents of at least about 6 weight percent and as much as 13 weight percent at an optimum level wherein the coals or blends of coals are sprayed with an amount of an aqueous dispersion containing a non-ionic surfactant characterized by an HLB number between about 4 and 18.